Outboard motors are typically self-contained propulsion units that are mounted on the transom of a watercraft. Since their invention in the beginning of the 20th century—see for example U.S. Pat. No. 1,524,857 to Ole Evinrude—they have existed in many configurations. Examples of current day configurations are those motors marketed under the trademark Evinrude® (www.evinrude.com) by BRP US Inc. of Sturtevant, Wis., the assignee of the present application.
The design of engines of outboard motors differs from the design of engines of other automotive and recreational vehicles (i.e. snowmobiles, all-terrain vehicles, motorcycles, karts, personal watercraft, etc.) in several significant ways. Firstly, owing to their self-contained nature, outboard motors typically include an internal combustion engine, a transmission including a series of shafts and gears, a propulsion device (typically a propeller), structural housing for all of these parts, and external fairings covering the unit. They are mountable on and removable from the watercraft as a complete unit, and in some cases they are portable. While in use, steering, tilt and trim of the watercraft are accomplished by moving and positioning the entire unit itself. While not in use the entire unit is typically tilted so as to be out of the water in which the watercraft is located. Several outboard motors may be mounted onto the same watercraft and be in concurrent use, meaning the above functions must be accomplished without one motor interfering with any of the other motors. For all of these reasons, as persons skilled in the art of their design have recognized, outboard motors, and particularly the internal combustion engines and associated components thereof, must generally be as small and as compact in their design as possible. Much development effort has been spent over time in attempting to ensure that this is the case. In this respect, see for example, U.S. Pat. No. 5,873,332 to Taue et al. assigned to Yamaha Hatsudoki K.K. (a competitor of the assignee of the present application) and U.S. Pat. No. 5,207,190 to Torigai et al. assigned to Sanshin Kogyo K.K. (a related-entity to Yamaha). Finally, for those motors having reciprocating piston internal combustion engines, the engines are arranged in the outboard motor such that the crankshaft is vertical. In no other common vehicle engine application does this occur.
Further complicating the design of such motors is the increasing desire of users thereof for more power output per motor. For example, Yamaha Motor Corp. is currently marketing a 350 hp (261 kW) outboard motor. In order to accommodate such power output requirements and still have a reasonably compact engine design, outboard motor manufacturers have switched to what are commonly known as “V-type” engines, i.e. engines having two cylinder banks—of 2 (a “V4”), 3 (a “V6”) or 4 (a “V8”) aligned cylinders each—that are at an angle (usually 60 or 90 degrees) to one another. In addition to the previously referred to Evinrude® and Yamaha® outboard motors, the aforementioned '190 and '332 patents provide examples of such engines as also used in outboard motors.
In order to increase the power output still further, the addition of more cylinders to a conventional engine is required. However, the use of V8 configuration and even larger V-type configurations such as the V10 or V12 generally yields outboard motors that are unwieldy in part in that they are too long from top to bottom, creating difficulties with respect to watercraft trim and in tilting the outboard motor out of the water when not in use. Furthermore, for 2 stroke outboard engines, the tuning of the exhaust wave is greatly reduced with cylinder banks having 3 or more cylinders. Consequently the engine is less efficient and necessitates even larger displacement to generate power output. Further, although other engine configurations are known from other engine applications (usually automotive—e.g. the multi-row V-type engine of U.S. Pat. No. 6,076,489 to Deutsch et al. and the W-type engine of U.S. Patent Application Publication No. 2001/0054396 to Hurt), these engines are also generally unwieldy for marine applications in part in that they are too wide, limiting the number of engines that may be concurrently placed on the transom of the watercraft. Further they are difficult to design air intake, air exhaust, and fuel systems therefor. Improvements in engine configuration for outboard motors, particularly for higher power output engines, are therefore desirable.
Further, as is well known in the art, the reciprocal motion of the piston of an engine is translated into the rotational motion of the engine's crankshaft by a connecting rod interconnecting the two. As a result, the connecting rod's motion has both a reciprocal component and a rotational component to it. When combustion occurs in the combustion chamber the piston is forced downward in the cylinder. Because of the relationship between the geometries of the piston, the connecting rod, and the crankshaft, and the conversion of the piston's motion into the crankshaft's motion, the downward-forced piston will also be forced against the cylinder wall on the side of the cylinder on which the connecting rod is located at that point in time in the crankshaft's motion. This force against the wall of the cylinder is known in the art as the major piston side thrust, but it will simply be referred to as “piston side thrust” for the purposes of the present application.
As is also known in the art, the walls of a cylinder are not equal in terms of temperature during operation. The portion of the cylinder wall on the side of cylinder on which the exhaust port is located (i.e. the exhaust side of the cylinder) is much hotter than the portion of the cylinder wall on the side of the cylinder on which the intake port is located (i.e. the intake side of the cylinder). This is the case as the exhaust gas is much hotter than the intake gas. As the heat on the exhaust side of the cylinder tends to greatly increase wear on piston and particularly the piston rings sealing the piston against the walls of the cylinder, ideally the piston side thrust for any given cylinder would be directed against the intake side of the cylinder (to avoid exacerbating the situation and to reduce the wear on the piston on every stroke).
This is not the case, however, in V-type engines employed in conventional outboard motors, given the need for a compact engine arrangement as described above. In such engines, either the intake or the exhaust manifold is located in the valley. For example, as shown in the '190 patent, the intake manifold and the intake sides of each of the cylinders are located in the valley between the cylinders or cylinder banks. This means that for the first cylinder or bank of cylinders in the direction of rotation of the crankshaft, the piston side thrust will be against the intake side of the cylinder(s). However, for the second cylinder or bank of cylinders in the direction of rotation of the crankshaft, the piston side thrust will be against the exhaust side of the cylinder(s). U.S. Pat. No. 4,184,462 shows an example of engine where the intake manifold and the intake sides of each of the cylinders are located away from the valley, and the exhaust manifold and the exhaust sides of each of the cylinders are located in the valley between the cylinders or cylinder banks. In that case, for the first cylinder or bank of cylinders in the direction of rotation of the crankshaft, the piston side thrust will be against the exhaust side of the cylinder(s). However, for the second cylinder or bank of cylinders in the direction of rotation of the crankshaft, the piston side thrust will be against the intake side of the cylinder(s).
Heretofore, this has simply been a situation that designs (and owners) of such engines have had to live with. Improvement would be desirable.
Therefore, there is a need for in the art for a marine outboard motor having an improved engine configuration.